Thermographic printing is an imaging process in which powdered resins are retained on a substrate by previously printed wet ink. Typically, the wet ink is either a solvent-containing ink wherein the solvent is capable of dissolving the powdered resin and wherein the ink, by itself, is capable of drying to form a permanent dry film on the substrate when the solvent is removed or a drying oil or oil modified alkyd which is capable of self drying by oxygen absorption and polymerization. The overall thermographic process includes printing the ink, applying powder resins over the ink, removing powder not retained by the ink, and baking or curing to form a composite coating of intermixed ink and powder. Coating thicknesses attainable by this process typically lie between 10 and 30 .mu.m.
Thermographic printing is used in the graphic arts for achieving a raised print effect. Ink is printed, usually by offset lithography, and the printed sheets are then passed through a powdering machine where powder is applied and fused onto the printed areas. This process can generally be carried out much faster than screen printing, which is an alternative method to obtain raised prints on the order of 25 .mu.m in thickness.
I have concluded that thermographic printing has potential advantages over screen printing in the manufacture of printed circuits due to the higher speed capability as compared with the screen printing methods presently employed in the manufacture of printed circuits. Further, thermographic printing is desirable due to the capability of producing patterns with narrower printed lines and spaces as compared with screen printing, which capability is essential in advanced printed circuit technology. Furthermore, the life of parts employed in thermographic printing, e.g., printing plate and blanket life, would be orders of magnitude greater than screen life. I have also found that thermographic coatings suitable for printed circuit manufacture can be made using solvent-free, low-cost inks and powders. The ability to employ solvent-free inks which are strippable by alkali solutions rather than organic solvents is of great importance in today's environmentally concerned society and in order to best meet governmental environmental regulations.
Consequently, it is desirable to develop materials and processes for thermographic etch and plating resists and solder masks for use in manufacturing printed circuit boards and other applications involving thermographic printing, which are strippable preferably without or with a minimal amount of organic solvents and which do not contain solvents in the ink. These materials and processes should also have the ability of being printable with normal printing techniques such as offset lithography, have adequate mechanical and chemical properties while resident on the substrate and should be removable at low cost. To be suitable for printed circuit manufacture, the ink should preferably be printable by dry offset printing to a typical printed thickness in the order of about 3 .mu.m. The powder should be free flowing in the particulate state and be compatible with the ink holding it. After baking, preferably at temperatures under 150.degree. C., the composite coating must cover completely the area to be protected and must be non-spreading so as to maintain fine line integrity. The coating should be adherent and strong enough to resist scratching and blocking during normal handling (blocking is the sticking together of two printed surfaces when pressed together at ambient temperature). Furthermore, the coating should have sufficient chemical resistance and not contaminate the plating or etching solution.
The use of powdered resin to cure solvent-free inks is generally known in the art. Among these are anti-offset powders which coat the entire printed surface and thereby provide spacing between adjacent printed sheets as described in U.S. Pat. No. 2,110,219. Also, the use of resin powders in thermographic printing processes has been described in U.S. Pat. No. 3,911,160. In the latter patent, the ink employed is either a drying oil vehicle ink, a drying oil alkyd ink, a resin-modified drying oil ink, gum rosin or tall oil rosin ink, a mineral oil or petroleum oil ink or an epoxy, polyurethane or polyamide modified ink. The inks described in this patent can be dried in the absence of powdered resin and the powdered resins utilized in combination with the inks may not give rise to a combination in which the resin is soluble in the ink as opposed to any organic solvent used in combination therewith. The requirement for an ink-powder combination useful in the manufacture of printed circuit boards is substantially more stringent than for many other general thermographic printing applications. One reason for this is the necessity of obtaining fine line spacing between adjacent lines on thermographic offset printed patterns for printed circuit boards, using ink-powder combinations which are strippable. This at least adds the requirement that the powder-ink combination cannot substantially spread during the heating operation so that fine-line differentiation such as a 15 mil or narrower line with 15 mil or narrower spacings can be maintained.